The present invention relates to the production of carbon fibers from carbon-containing precursor fibers such as polyacrylonitrile fibers, and particularly to methods and systems for processing such precursor fibers to provide high tensile carbon fibers with improved yield and uniformity.
A variety of methods have been employed for producing carbon fibers by first oxygenating and then carbonizing precursor fibers, such as polyacrylonitrile fibers, in an inert atmosphere. Most methods keep the fibers under tension, as by restraint against shrinkage, during at least some of the process steps. Tension during oxidation, also called stabilization, is a precondition to obtaining the levels of tensile strength and modulus of elasticity that are desired in the final product. Many variants have been employed in the carbonization phase, which takes the oxidized fibers to a higher, final temperature level within a relatively short time, using a nitrogen or other inert gas as the environment. Carbonization has most often been carried out with single stage furnaces, but multiple stages have also been used. Elongation and restraint against shrinkage have been employed, generally in one stage. Although the material used is sometimes in fabric form, the typical process utilizes large tows, with multiple filaments being distributed across a flat plane so that longitudinal tension can be exerted and the gases have substantially equal access to the fibers.
Illustrative of variations in the above noted procedures for producing carbon fibers are U.S. Pat. Nos. 3,652,22 3,663,173 and 3,716,331, which deal with the use of multiple carbonization stages and the use of tension during carbonization, but all are concerned with partially carbonized cellulosic precursors. Restraint against shrinkage is used with polyacrylonitrile fibers during carbonization in U.S. Pat. Nos. 3,698,865 and 3,412,062. In U.S. Pat. No. 4,100,004 a two stage oxygenation procedure is disclosed together with a two stage carbonizing procedure, employing temperatures in the range of 600.degree. to 700.degree. C. in the first carbonizing furnace and a temperature in the range of 1050.degree. to 1600.degree. C. in the second furnace.
A Japanese publication No. J5-4147-222 discloses a process for producing carbon fiber with improved tensile strength and modulus by first passing acrylic fibers through an oxidizing oven at 230.degree.-250.degree. C. to effect 10% shrinkage. The flameproofed or stabilized fibers are then preliminarily carbonized at a temperature from 300.degree. to 800.degree. C., particularly from 400.degree. to 600.degree. C. while being subjected to a high stretch up to 25%, in a nitrogen gas atmosphere. The elongated partially carbonized fibers thus obtained are finally or completely carbonized at elevated temperature of 1300.degree. C. with 3% shrinkage. This is a specific example of the multiple stage carbonization techniques mentioned above. The use of multiple stages slows the outgassing or decomposition process somewhat, reducing defects in the carbon fibers.
More recently in the development of this art, workers have confronted the secondary but important problems arising from the release of volatile components and tars in the carbonization environment. It has been recognized that redeposited tars and other matter accumulate and restrict the flow of gases, and further that contact of this matter with the fibers damages or weakens them. Yields are not only decreased but the entire process is unduly sensitive to operating conditions. Consequently, as shown by various publications, different expedients have been proposed for alleviation of problems arising from the products of decomposition. Examples of these approaches are found in U.S. Pat. No. 3,508,871 (using a solvent to remove tarry materials), Japan Kokai No. 7740622 (two stage carbonization), German Offen. No. 2133887 (fast carbonization using electric oven and volatiles removal), U.S. Pat. No. 4,020,273 (upward flow of gas in opposition to downward flow of fibers) and U.S. Pat. No. 4,073,870 (countercurrent flow of gas in a two section furnace).